Circulating tumor cells (CTCs) pose continuous and persistent threats to create new metastases albeit at an unknown, extremely low efficiency. Compared to single CTCs, clusters of multicellular CTCs possess 20-100 times higher metastatic capacity, create more polyclonal metastasis, and correlate with worse prognosis. Our previous studies identified that in addition to collective migration and cohesive shedding, tumor cell aggregation is a new mechanism for CTC cluster formation, enhancing cancer stemness and polyclonal metastasis. The objectives of this proposal are to elucidate the molecular mechanisms underlying CD44 and PAK2-promoted CTC aggregation, identify the drivers of CTC cluster-mediated polyclonal metastasis, and therefore develop proof-of-principle targeting strategies to block lung metastasis of triple negative breast cancer, using multiple human PDXs and CTC lines as well as mouse tumor models. The collaborative team includes Dr. Huiping Liu (Northwestern University) with expertise in CTC and cancer stemness, breast oncologist Dr. Massimo Cristofanilli (Northwestern University), imaging expert Dr. Constadina Arvanitis (Northwestern University), and bioinformaticist and structural biologist Dr. Yang Shen (Texas A & M).